Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at intervals measured during a section. The samples are analyzed to determine their detrital residual magnetism (DRM), i.e. the polarity of the Earth`s magnetic field at the time of layer deposit. For sedimentary rocks, this is possible because very fine-grained magnetic minerals (< 17 μm) behave like tiny compasses that orient themselves towards the Earth`s magnetic field as they fall through the water column. At funerals, this orientation is maintained. In volcanic rocks, magnetic minerals that form in the molten state orient themselves towards the surrounding magnetic field and are fixed during the crystallization of the lava. A gap or missing layers in the geological record of a region is called a stratigraphic hiatus. This may be the result of stopping sediment deposition. Alternatively, the deviation may be due to removal by erosion, in which case it may be called a stratigraphic vacuum. [2] [3] This is called a pause because the deposit has been suspended for a period of time. [4] A physical deviation can represent both a period of non-deposition and a period of erosion. [3] A geological fault can cause a pause to appear.
[5] The Catholic priest Nicolas Steno created the theoretical basis of stratigraphy when, in 1669, in an article on the fossilization of organic remains in sedimentary layers, he introduced the law of superposition, the original principle of horizontality and the principle of lateral continuity. Biostratigraphy or paleontological stratigraphy is based on fossil evidence in rock layers. Layers of extended sites containing the same fossil fauna and flora should be correlated over time. Biological stratigraphy was based on William Smith`s principle of faunal succession, which preceded biological evolution and was one of the earliest and most powerful sources of evidence for it. It provides strong evidence of species formation (speciation) and extinction. The geological time scale was developed in the 19th century, based on evidence of biological stratigraphy and faunal succession. This time scale remained a relative scale until the development of radiometric dating, based on an absolute time frame, which led to the development of chronostratigraphy. An important development is the Vail curve, which attempts to define a global historical sea level curve based on inferences from global stratigraphic models. Stratigraphy is also commonly used to delineate the nature and extent of reservoir rocks containing hydrocarbons, seals and traps of petroleum geology. The variation of rock units, which is more clearly represented by visible stratification, is due to physical contrasts in the rock type (lithology).
This variation can occur vertically as stratification (litter) or laterally and reflects changes in deposition environments (known as facies change). These variations provide lithostratigraphy or lithological stratigraphy of the rock unit. Key concepts in stratigraphy are to understand how certain geometric relationships between rock layers are formed and what these geometries say about their original deposition environment. The basic concept of stratigraphy, the so-called law of superposition, states: In an undistorted stratigraphic sequence, the oldest layers are at the base of the sequence. Cyclonic stratigraphy documents often cyclical changes in relative proportions of minerals (especially carbonates), grain size, thickness of sedimentary layers (varvae) and fossil diversity over time associated with seasonal or longer-term changes in paleoclimate. The first practical application of large-scale stratigraphy was made by William Smith in the 1790s and early 19th century. Known as the “father of English geology,”[1] Smith recognized the importance of rock strata or stratification and the importance of fossil markers for layer correlation; he created the first geological map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart, who studied the geology of the area around Paris. Stratigraphy is a branch of geology that deals with the study of rock layers (layers) and stratification (stratification). It is mainly used in the study of sedimentary and stratified volcanic rocks.
Stratigraphy has three related subfields: litostratigraphy (lithological stratigraphy), biostratigraphy (biological stratigraphy) and chronostratigraphy (age stratigraphy). This technique is used to date sequences that typically lack fossils or igneous rocks. The continuous nature of sampling means that it is also a powerful technique for estimating sediment accumulation rates. Chimostratigraphy studies changes in the relative proportions of trace elements and isotopes within and between lithological units. Carbon and oxygen isotopic ratios vary over time, and researchers can use them to map subtle changes that have occurred in the paleoenvironment. This led to the special field of isotopic stratigraphy. Oriented paleomagnetic cores are collected from the field; Mudstones, siltstones and very fine-grained sandstones are the preferred lithologies, as magnetic grains are finer and are more oriented towards the surrounding field during deposition. If the old magnetic field were aligned in the same way as the current field (magnetic north pole near the north pole of rotation), the layers would maintain a normal polarity. If the data indicate that the magnetic north pole is close to the south rotation pole, the layers would have an opposite polarity. Chronostratigraphy is the branch of stratigraphy that places an absolute age and not a relative age on rock layers.
The branch deals with the derivation of geochronological data for rock units, both directly and inferentially, so that a sequence of weather-related events that caused the rock formation can be derived. The ultimate goal of chronostratigraphy is to provide data on the order of deposition of all rocks in a geological region and then for each region, and more broadly to provide a complete geological record of the Earth. The results of the individual samples are analyzed by removing natural residual magnetization (NRM) to expose the DRM. After statistical analysis, the results are used to generate a local magnetostratigraphic column, which can then be compared to the global magnetic polarity time scale.